Light emitting devices (LEDs) are an important class of solid state devices that convert electric energy to light and commonly comprise an active layer of semiconductor material sandwiched between additional layers. As the quality of semiconductor materials have improved, the efficiency of LEDs has also improved. Commercially-available LEDs are being made from alloys of indium, aluminum, and gallium with nitrogen (AlInGaN). These alloys make possible LEDs which operate in the ultra-violet to green spectral regions. However, the efficiency of LEDs is limited by their inability to couple all of the light that is generated by an active layer out of the LED chip. When an LED is energized, light emitting from its active layer (in all directions) reaches the LED surfaces at many different angles. Typical semiconductor materials have a high index of refraction compared to ambient air (n=1.0) or encapsulating epoxy (n≈1.5). According to Snell's law, light traveling from a material having an index of refraction, n1, to a material with a lower index of refraction, n2, at an angle less than a certain critical angle θC relative to the surface normal direction will cross to the lower index region, whereθC=sin−1(n1/n2)  (1)
Light that reaches the semiconductor surface at angles greater than θC will experience total internal reflection. This light is reflected back into the LED chip where it can be absorbed within the chip or in metal contact layers that are attached to the chip. For conventional LEDs, the vast majority of light generated within the structure suffers total internal reflection before escaping from the semiconductor chip. In the case of conventional GaN-based LEDs on sapphire substrates ˜70% of the emitted light is trapped between the sapphire substrate and the outer surface of the GaN. This light is repeatedly reflected, greatly increasing its chance for reabsorption and loss.
Several techniques have been described to improve light extraction from LEDs. Providing the device with reflective contacts is one such technique. This improves LED efficiency because light that is trapped within the structure and is incident on the contact metals will be reflected back into the device rather than being absorbed. This allows the light to have another opportunity to escape the chip the next time it is incident on the LED surface. While reflective contacts improve light extraction, conventional LEDs still suffer from significant absorption losses. Roughening the top surface is another technique to improve light extraction. Roughening scatters, or sometimes randomizes, the angle of reflected light so that trapped light is redirected. This prevents light from being repeatedly reflected by parallel interfaces. Some of the scattered light then has an opportunity to strike a surface within the critical angle for internal reflection before being absorbed. Typical semiconductor layers are thin so only fine-scale roughening is usually possible. Also, roughened surfaces can cause other problems with the LED fabrication process. For example, contacts to roughened surfaces can be problematic. Also, roughened surfaces can cause it to be difficult to align photomasks to the wafer. And they make it difficult for the pattern recognition equipment that are used to bond and inspect the wafers to work properly. Therefore another technique to redirect trapped light is desirable. Another technique to scatter trapped light is to provide a rough interface between the GaN and the underlying substrate. This can be done by patterning and roughening the substrate prior to the growth of the semiconductor layers. This technique is effective at improving light extraction; however, the textured surface of the substrate affects the subsequent growth of the semiconductor layers. The quality of the semiconductor layers is often adversely affected, and the reproducibility of the growth is poor.
Additional methods of improving light output efficiency are reviewed in U.S. Pat. No. 6,657,236 which is included herein in its entirety by reference. U.S. Pat. No. 6,657,236 and U.S. Pat. No. 6,821,804 teach another method requiring a first spreading layer of a n type doped AlInGaN based material; a second spreading layer is preferably a thin, semi-transparent metal such as Pd, Pt, Pd/Au, Ni/Au, NiO/Au or some combination thereof deposited on, preferably, a p-type AlInGaN surface. Light extraction structures are then fashioned as arrays of light extraction elements or disperser layers. The light extraction elements are formed from a material having an index of refraction higher than the devices encapsulating material.
U.S. Pat. No. 6,831,302 teaches a structure comprising a multi-layer stack of materials, a layer of reflective material capable of reflecting at least about 50% of light impinging thereon and wherein a surface of a n-doped material, such as n-GaN, has a dielectric function that varies spatially according to some pattern. U.S. 2005/0227379 teaches shaping a surface of a semiconductor layer with a laser to improve the light extraction efficiency. Alternatively a substrate may contain three dimensional geometric light extraction patterns or a light emitting element on a substrate contains at least one layer with a pattern to produce light extraction features.
All of the prior art suffer from marginal improvement of light extraction efficiency or high manufacturing cost or both. A simple solution is needed which improves the overall light delivered from a light emitting device at a low cost.